A quantitative study of synchysis senilis of vitreous and a statistical evaluation of its relationship to posterior vitreous detachment (PVD) was performed in eyes of 2,246 autopsied subjects (4,492 eyes); cases in which there was any cause for synchysis or PVD except senescence were excluded. Synchysis was graded 0 through 6 according to degree of destruction as judged by the technique of suspension-in-air; synchysis increased with age (P less than 0.00005). PVD was also age-related, becoming notable in the seventh decade (27%) and reached a zenith in the eighth (63%). When the rate of PVD for each grade of synchysis was plotted, there was a significant upturn between synchysis grades 3 (50% destruction) and 4 (67%) (P less than 0.00005); this trend was also evident when age was considered. These quantitative data reaffirm the role of synchysis in rhegmatogenous PVD and suggest that the vitreous of human eye can tolerate only a given degree of synchysis before some event, possibly related to vitreous instability, initiates PVD.